The present invention is related to cancelling supply noise in a phase-locked loop, and in particular to a charge pump buffer providing supply noise cancellation for a phase-locked loop.
Phase-locked loops (PLLs) are used to provide a clock signal that tracks the frequency and phase of an input reference frequency. Although various types of PLLs are available, one particular example uses a voltage controlled oscillator (VCO) to generate an output clock. The frequency of the output clock may be adjusted to match the input reference frequency or some multiple of the input reference frequency. A phase frequency detector in the PLL compares the input reference frequency with the output clock and, together with a charge pump in the PLL, generates a voltage that controls the frequency at which the VCO oscillates. In one particular type of VCO, the control voltage adjusts the capacitance in an LC tank circuit in the VCO. The LC tank circuit includes a voltage controlled capacitor and an inductor that alternately charge and discharge, and this oscillation generates the output clock. By adjusting the capacitance in the LC tank circuit, the time it takes to charge and discharge the LC tank circuit changes. For example, by decreasing the control voltage, the capacitance of the LC tank circuit increases and the frequency decreases. By increasing the control voltage, the capacitance of the LC tank circuit decreases and the frequency increases. Thus, if the phase frequency detector determines that the output clock from the VCO is slower than the reference frequency, the phase frequency detector will cause the charge pump to increase the control voltage to the VCO to increase the frequency of the output clock. If the phase frequency detector determines that the output clock from the VCO is faster than the reference frequency, the phase frequency detector will cause the charge pump to decrease the control voltage to the VCO to increase the frequency of the output clock.
This voltage control of the frequency in a PLL provides a simple and effective way to tune the PLL. However, voltage control renders the PLL susceptible to noise on the power supply. For example, if the supply voltage used to charge the LC tank circuit in the VCO changes rapidly due to noise, the time it takes to charge the LC tank circuit also changes and causes jitter in the output frequency from the PLL. In extreme cases, supply noise can even cause a PLL to lose its lock with the reference frequency.
Hence, for at least the aforementioned reasons, there exists a need in the art for systems and methods for cancelling phase-locked loop supply noise.